Occurrence characteristics of the movable fluid in heterogeneous sandstone reservoir based on fractal analysis of NMR data: A case study of the Chang 7 Member of Ansai Block, Ordos Basin, China

Abstract

In the Ordos Basin, Chang 7 sandstone is considered an essential tight sandstone reservoir. In this research, the occurrence characteristics of the movable fluid, fractal characteristics, and their relationship in Chang 7 reservoir of the Ansai Block, Ordos Basin, have been quantitatively evaluated through nuclear magnetic resonance (NMR) and fractal theory. Moreover, the factors influencing the fractal dimensions and movable fluid distribution have been determined through a series of experiments, i.e., conventional physical property, cast thin section, scanning electron microscope (SEM), high-pressure mercury intrusion (HPMI), and X-ray diffraction technique (XRD). It has been noticed the movable fluid distribution in the tight sandstone reservoirs is greatly affected by the pore structure heterogeneity. Results show the main pore types in Chang 7 are the intergranular dissolution pores, intragranular dissolution pores, residual intergranular pores, and inter-crystalline pores. The pore size chiefly ranges from 0.1 nm–5 μm, while the main fluids storage spaces are nanopores and submicron pores. The reservoir has been divided into three types (type Ⅰ, Ⅱ, and Ⅲ) based on the distribution of the T2 spectrum and petrophysical parameters. Moreover, these reservoirs have various movable fluid distributions and fractal structures. Multiple factors such as pore structure, pore size distribution, and mineral compositions are discussed, which influence the fractal dimension and movable fluid saturation. The reservoirs having more nanopores are often associated with strong heterogeneity and low movable fluid content. The fractal dimension is weakly affected by pore structure parameters and mineral compositions. The movable fluid saturation is significantly affected by pore structure while weakly affected by mineral compositions. The relationship between heterogeneity and movable fluid distribution varies with reservoir types. The inhibition of heterogeneity on the movable fluid of different reservoirs is mainly reflected in various pores. Movable fluids controlled by throats are primarily distributed in different pores, resulting in diverse effects of heterogeneity on controlling the effect of different throats in various reservoirs. The findings of this research can help better understand the occurrence characteristics of the movable fluid in tight sandstone reservoirs with different fractal features.